Provide large enough buffer in test_util_sscanf()

This was causing crashes during unit test runs, as stack smashing
protections got triggered. Issue spotted by weasel

changes/bug5449

@@ -0,0 +1,6 @@
+  o Minor bugfixes:
+    - In the testsuite, provide a large enough buffer in the tor_sscanf
+      unit test. We'd otherwise overrun that buffer and crash during the
+      unit tests. Fixes bug 5449; bugfix on 0.2.3.12-alpha. Thanks weasel
+      for spotting the bug.
+

src/test/test_util.c

@@ -1376,7 +1376,7 @@ static void
 test_util_sscanf(void)
 {
   unsigned u1, u2, u3;
-  char s1[10], s2[10], s3[10], ch;
+  char s1[20], s2[10], s3[10], ch;
   int r;
 
   /* Simple tests (malformed patterns, literal matching, ...) */